The regulated sorting of plasma membrane proteins is controlled by extracellular signals. A number of membrane proteins are retained intracellularly until their transport to the plasma membrane is stimulated by a particular signal. The transport of the S. cerevisiae general amino acid permease Gaplp is regulated in this manner. Under nutrient-limiting conditions, Gaplp is transported to the plasma membrane where it has high activity. Conversely, growth in nutrient-rich conditions results in little Gaplp at the plasma membrane, and low permease activity. Recently, a novel complex containing two small GTP-binding proteins has been shown to be required for Gaplp plasma membrane localization and activity (Gao and Kaiser, manuscript in preparation). My aims are to: (1) Determine the intrinsic and activated guanine nucleotide binding and enzymatic properties of the GTPases, (2) Identify additional components of this Gaplp-sorting complex using biochemical and genomic means, and (3) Reconstitute the endosome-to-Golgi transport of Gaplp in vitro using purified membrane fractions and components, with the goal of identifying the requirements for this process. [unreadable] [unreadable]